The present invention relates to preparation of a herbicidal formulation useful in agriculture and in other situations where control of weeds or other vegetation is desired. In particular, it relates to preparation of a herbicidal active ingredient, namely N-phosphonomethylglycine (glyphosate) in the form of the ammonium salt thereof, as an intermediate useful in further processing together with a surfactant to make a water-soluble or water-dispersible dry particulate herbicidal formulation.
Glyphosate herbicides, especially herbicides comprising a water-soluble salt of glyphosate, are well known. Specifically, the monoammonium salt of glyphosate is disclosed as a useful herbicide for example in U.S. Pat. No. 4,405,531 to Franz. Unless the context demands otherwise, xe2x80x9cammonium glyphosatexe2x80x9d herein refers to the monoammonium salt of glyphosate, which has the chemical formula 
it being understood that the mole ratio of ammonium cations to glyphosate anions in such a salt is not necessarily exactly 1. A slight molar excess of either ammonium cations or glyphosate anions, for example providing a mole ratio of about 0.8 to about 1.25, is not inconsistent with the term xe2x80x9cammonium glyphosatexe2x80x9d as used herein.
Ammonium glyphosate is the primary salt of choice in the preparation of dry glyphosate herbicide formulations. A xe2x80x9cdryxe2x80x9d formulation herein is a composition that is solid, usually particulate, wherein particles are either aggregated as in a granular composition or non-aggregated as in a powder. The word xe2x80x9cdryxe2x80x9d in this context does not imply that the formulation is necessarily free of water or other liquid, only that it is dry to the touch. Dry formulations can contain up to about 5% by weight of water, but more typically the water content is less than about 1%, for example about 0.5% or lower.
Dry formulations of glyphosate herbicides, like the corresponding liquid (normally aqueous) formulations, typically contain one or more surfactants in addition to the glyphosate salt. Surfactants are important components of glyphosate formulations because, when a glyphosate formulation is diluted, dissolved or dispersed in water for application by spraying to foliage of plants, the surfactants assist in retention of droplets of the spray by the foliage, adhesion of the spray droplets to the foliar surface and penetration of the glyphosate through the hydrophobic cuticle that covers the foliar surface, by these means and possibly in other ways enhancing herbicidal effectiveness of the glyphosate spray. Specific surfactant types differ greatly in the degree to which they enhance herbicidal effectiveness of glyphosate, and it is therefore important to select a suitable surfactant or combination of surfactants, as demonstrated by Wyrill and Burnside, Weed Science 25, 275-287, 1977.
The optimum amount of surfactant for delivering the desired herbicidal effectiveness is typically in the range of about 0.2 to about 1 part by weight of surfactant per part by weight of glyphosate, expressed as acid equivalent (a.e.). When it is desired to formulate the glyphosate herbicide in dry form, it can be difficult to load such an amount of surfactant into the formulation without the formulation becoming sticky, having a tendency to cake or lacking good pouring or flow properties.
At least three approaches are known in the art to overcoming the problems of providing a sufficient amount of surfactant in a dry glyphosate formulation. The first and most straightforward is to add an inert particulate carrier that can absorb or adsorb the surfactant to a sufficient degree to avoid the problems mentioned above. The carrier can be insoluble but dispersible in water, as in the case for example of particulate silica, or it can be soluble in water, as in the case for example of ammonium sulfate. However, the addition of such a carrier inevitably reduces the maximum loading of glyphosate herbicide that can be accommodated in the formulation and for this reason adds substantially to the cost per unit of glyphosate a.e. of the resulting formulation. In this regard it should be recognized that the cost of processing is a significant element in the cost of a dry formulation, and the cost of processing is dictated by the volume of product produced. A product that has to be produced in high volume because its loading of active ingredient is low therefore suffers a significant penalty in cost per unit of active ingredient.
A second approach, as illustrated by U.S. Pat. No. 4,931,080 to Chan and Djafar, is to select a surfactant that is solid at ambient temperature. In this approach the surfactant is melted before mixing with particulate glyphosate herbicide and water, so that upon drying and cooling the surfactant solidifies to form a matrix surrounding the herbicide particles. There is no need for an inert carrier. Unfortunately the list of surfactants that are solid at ambient temperature excludes many surfactants that are among the most effective in potentiating glyphosate herbicidal activity.
A third approach, therefore, as illustrated by U.S. Pat. No. 5,656,572 to Kuchikata et al. (the ""572 patent), is to select a surfactant that is liquid at ambient temperature and to ensure that the glyphosate herbicide particles themselves absorb or adsorb a sufficient amount of surfactant to avoid the problems of stickiness, caking and poor flowability. The ""572 patent teaches that this can be achieved most readily if the surfactant selected is one that gels when added to water. However, it is also clear that the absorption and/or adsorption properties of the glyphosate herbicide particles greatly influence the amount of a liquid surfactant that can be included in a formulation.
Ammonium glyphosate is the preferred salt for use in preparing dry glyphosate formulations for a number of reasons, but perhaps mainly for the reason that ammonium glyphosate is relatively non-hygroscopic. Salts favored for preparation of aqueous formulations, such as the isopropylammonium salt or the trimethylsulfonium salt, are very difficult to dry down to a crystalline state and, once dry, have a strong tendency to reabsorb water. The sodium salt, disclosed to be useful in dry glyphosate herbicide formulations for example in International Patent Application No. WO 87/04595, is much less hygroscopic than these salts but nonetheless requires packaging with a very water-impermeable material to avoid absorption of water vapor from the atmosphere and consequent loss of free-flowing properties. U.S. Pat. No. 5,324,708 to Moreno et al. discloses a process for preparing a non-hygroscopic monoammonium glyphosate; however, dry ammonium glyphosate prepared by any known process is adequately non-hygroscopic for most practical purposes.
U.S. Pat. No. 5,266,553 to Champion and Harwell discloses a process for preparing a dry water-soluble composition comprising a salt of a herbicidal compound that includes a carboxylic acid functionality. This process comprises forming an aqueous solution or slurry of such a salt by reacting the herbicidal compound with a neutralizing base in the presence of water, and thereafter removing the water to provide the dry salt. The process is directed particularly at substituted benzoic acid herbicides and phenoxy-substituted carboxylic acid herbicides, but is said to be useful also for glyphosate. As indicated below, dry ammonium glyphosate powder prepared by a slurry reaction process has proved generally unsuitable for downstream formulation with surfactant.
Commercial herbicides in the form of dry water-soluble granules containing ammonium glyphosate together with a liquid surfactant include Roundup(copyright) Dry, Roundup(copyright) Max and Rival(copyright) herbicides, marketed by Monsanto Company in several countries.
Numerous granulation processes have been disclosed that are suitable for preparing water-soluble or water-dispersible granules of ammonium glyphosate with a liquid surfactant. One such process is pan granulation. However, a more widely used granulation process for a dry ammonium glyphosate formulation is extrusion granulation. An example of such a process is one that is broadly as described in British Patent No. 1 433 882 (xe2x80x9cthe ""882 patentxe2x80x9d), except that the primary active ingredient, namely ammonium glyphosate, is water-soluble rather than water-insoluble as in the process of the ""882 patent. In this illustrative process, ammonium glyphosate is mixed with surfactant and a small amount of water to form an extrudable wet mix, which is then extruded to form strands of extrudate that break spontaneously at the point of extrusion or shortly thereafter to form short cylindrical granules, which are then dried. Drying is preferably conducted in a fluid-bed dryer. The amount of water in the wet mix is critical to the operation. If the mix is too wet, the strands of extrudate do not readily break to form discrete granules. In addition, the extrudate strands tend to stick together and/or form lumps. If the mix is too dry, the resulting granules are friable and tend to generate a significant amount of fine particulate material during drying or later, during handling of packaged granules. Optionally a rolling or tumbing step can be inserted between extruding and drying, as taught in U.S. Pat. No. 5,443,764 to Lloyd, to improve uniformity of granule size and shape.
The process by which the ammonium glyphosate, used as an intermediate in making the finished formulation, is prepared has been found to affect to a great degree the absorptive and/or adsorptive properties of particulate ammonum glyphosate with respect to a liquid surfactant. The absorbency and adsorbency properties of the ammonium glyphosate particles are especially important where, as is desirably the case, an extrusion process such as that disclosed in the above-referenced ""882 patent is to be used in preparing the finished formulation.
Solid-state reaction of glyphosate acid and ammonium bicarbonate, as disclosed for example in the above-referenced ""572 patent, tends to produce a particulate ammonium glyposate having sufficient absorbency and/or adsorbency to permit satisfactory formulation with up to about 25% by weight of a liquid surfactant such as polyoxyethylene tallowamine. By contrast, reaction of an aqueous slurry of glyphosate acid with anhydrous ammonia or aqueous ammonia (ammonium hydroxide) tends to produce relatively non-absorptive or non-adsorptive ammonium glyphosate particles.
Because anhydrous and aqueous ammonia are much lower-cost sources of the ammonium cation than ammonium bicarbonate, numerous efforts have been made to develop processes wherein glyphosate acid is reacted with anhydrous or aqueous ammonia, yet wherein the resulting ammonium glyphosate is suitable for downstream formulation with surfactant, especially a liquid surfactant. To date, success has been achieved only when the reaction occurs in the presence of very small amounts of water, for example about 7 parts or less by weight of water per 100 parts by weight of dry ingredients. U.S. Pat. No. 5,614,468 to Kramer et al. discloses such a process wherein solid particulate glyphosate acid is reacted with aqueous ammonia, and U.S. Pat. No. 5,633,397 to Gillespie et al. discloses such a process wherein solid particulate glyphosate acid is reacted with anhydrous ammonia gas.
Processes wherein the acid-base reaction takes place in an aqueous medium, generating a concentrated aqueous solution of ammonium glyphosate, are easier to control than the above solid-state processes. In addition, the exothermic nature of the reaction gives rise to a need for dissipation of heat, which presents much fewer problems in an aqueous medium because of the much greater ease of ensuring adequate mixing and thereby heat exchange than is possible in a solid-state process. There is therefore a long-felt need in the art to develop an aqueous slurry process for making ammonium glyphosate wherein the ammonium glyphosate produced is suitable for downstream formulation with sufficient surfactant, in particular sufficient liquid surfactant, to provide a dry powder or granular formulation having a high degree of herbicidal effectiveness yet exhibiting good properties of storage without caking and good properties of flowability.
The present invention provides just such a process.